Brits in Northern Ireland certainly use the term “Republican” to describe the IRA - and indeed the whole of the nationalist side of the great divide. In mainland Britain, its not so common. Let us say that if a mainland Brit was describing that particular strand of political opinion, it wouldnt be the term that would spring to their minds first. Most would say “nationalist”, or even “catholic” (even though not all catholics support the idea of a united Ireland). They might use the term “fenian” (especially in Scotland). After that, the expressions used become a bit more, shall we say, perjorative? :)
Thank you for the info :)